1. Field of the Invention
The present invention relates to a process for the manufacture of SC-A paper having a high gloss and high smoothness, wherein the paper arriving from the paper machine is supplied on-line to a supercalender (multinip soft calender), wherein the paper travels through a plurality of nips for achieving the desired gloss and smoothness properties.
The present invention also relates to an apparatus for carrying out the process.
2. Description of the Related Art
Gloss and smoothness are characteristics of a paper web which not only influence the appearance of the paper web but also the possibilities of further processing of the paper web. In certain types of applications, gloss and/or smoothness values are desired which are to be reproducible as uniformly as possible.
Gloss and smoothness of the paper web are usually increased by conducting the paper web following the dry end of the paper machine into a smoothing unit which is composed of one or more nips, wherein the gloss and/or the smoothness of the paper web is increased by the pressure in the nip and the temperature of the rolls forming the nip. However, this makes it only possible to influence the gloss and smoothness of the paper web to a limited extent because, when the pressure in the nip is increased too much, the paper web is compressed too much and a volume loss of the paper web occurs. In this connection, there is the danger that the paper web loses stability. There are also limits with respect to the increase of the roll temperature because this increase requires a large amount of energy. For example, for achieving roll temperatures of 200.degree. C., significant quantities of energy must be supplied because the rolls are continuously cooled by the paper web travelling past the rolls.
Therefore, it has already been attempted, for example, in silicon papers, to influence the gloss and smoothness of the paper web by the moisture of the paper web. However, this has the disadvantage that after the treatment the supplied moisture has to be removed at least partially which requires additional process steps which, in turn, increase the time required for the treatment of the paper web and the apparatus is more complicated.
Basically, there are essentially two types of smoothing units. So-called supercalenders have a plurality of rolls arranged one above the other and nips provided between the rolls through which the paper web travels. The large number of nips produces a high degree of overlap or contact and a good distribution of the glazing work between pressure and temperature. Supercalenders are usually provide off-line, i.e., the paper web arriving from the paper machine is initially wound onto a reel-spool and is transferred together with the reel-spool to the supercalender, wherein the paper web travels through the supercalender at a significantly lower speed than the paper machine speed. The off-line installation has the advantage that the paper web can level out or equalize prior to entering the supercalender, so that the operation in the supercalender does not have to be carried out under the requirements of the paper machine which is influenced by many factors. However, the installation requirements are significantly higher. A supercalender classically has heated steel rolls, on the one hand, and paper rolls or rolls covered with cotton, on the other hand. More recently, also so-called multinip soft calenders are used in which the paper rolls are replaced by rolls having polymer covers. These rolls have a different elastic behavior than the paper rolls, so that it is possible to operate with a lower nip pressure.
Secondly, there are so-called machine calenders or soft calenders which can be connected on-line to a paper machine and through which, consequently, the paper web travels with the paper machine speed. However, machine calenders only have a small number of nips, so that the operation is carried out with higher pressure and temperature and, thus, the paper web is stressed to a greater extent. A significant disadvantage of the soft calenders is that not all types of paper can be upgraded to high qualities. In particular, it is not possible to manufacture highly compacted SC-A paper on-line in a soft calender. It was possible recently to achieve the printing properties of a natural gravure paper supercalendered in 11 nips in a soft calender with only four nips; however, this requires relatively high roll temperatures and compressive stresses in the nips. Also, these qualities can only be achieved in a range of speeds which corresponds to the glazing speed in the supercalender which is usual for this paper (see: Rothfuss, Ulrich: Inline- und Offline-Satinage von holzhaltigen, tiefdruckfahigen Naturdruckpapieren in: Wochenblatt fur Paperfabrikation 1993, No. 11/12, pages 457-466). Consequently, such qualities can only be achieved with the off-line installation of the soft calender.